1. Field of the Invention
The present invention relates, in general, to vacuum cups and, more specifically, to control apparatus for applying and releasing a vacuum from a vacuum cup.
2. State of the Art
Vacuum cups are commonly employed as workpiece gripping elements to engage and transport a workpiece in a manufacturing operation, to load and unload sheet metal parts into and from a die, or to carry a part, such as an automobile windshield, to the vehicle to which it is to be installed.
Such vacuum cups employ a control apparatus which uses a venturi passage in a body which is connected to a source of air under pressure. Air flow through the venturi passage induces a subatmospheric pressure in the throat of the venturi and in a passage connecting the venturi throat to the interior of the vacuum cup which will induce a vacuum within the cup when the cup engages a workpiece surface. In earlier versions of such vacuum cup control apparatus, it was necessary to constantly maintain the flow of air through the venturi passage in order to maintain the vacuum in the cup because the air withdrawn from the cup flowed into the venturi passage and to the discharge vent at the end of this passage. Upon cessation of the air flow, air at atmospheric pressure was free to flow in a reverse direction through the discharge vent, the venturi passage and into the vacuum cup to dissipate the vacuum therein.
Efforts to address this problem led to the use of one-way check valves between the venturi and the vacuum cup to seal the vacuum in the vacuum cup upon cessation of air flow through the venturi. This enabled the vacuum cup to securely maintain a grip on a workpiece after air flow through the venturi was stopped.
Another problem with previously devised vacuum cups deals with the quick release of the vacuum from the cup to release the workpiece at the conclusion of the handling operation. The passage from the discharge vent to the vacuum cup is a relatively restricted passage and the rate of air flow through this passage would diminish substantially as the pressure differential between the negative pressure within the cup and atmospheric pressure approached equalization. Further, even when the vacuum was turned "off", the suction remaining between the vacuum cup and the workpiece could be sufficient to cause the vacuum cup to remain attached to the workpiece and thereby prevent or make uncertain the release of the workpiece from the vacuum cup.
Efforts to overcome this problem led to the use of "blow-off" passages which inject air under pressure into the vacuum cup to rapidly release the vacuum between the vacuum cup and the workpiece. Despite these advances in the use of vacuum cups as work gripping elements, problems are still encountered. Leaks can occur in the vacuum cup or between the vacuum cup and the workpiece which cause a decrease in the vacuum in the vacuum cup and could lead to inadvertent separation of the workpiece from the vacuum cup. Such leaks could result from worn or cracked vacuum cups, deformed workpieces, etc. No apparatus has been devised for vacuum cups which detects leaks or a decrease in vacuum levels within a vacuum cup so as to enable vacuum to be reapplied to the vacuum cup to a preset level.
U.S. Pat. No. 4,750,768 discloses a gripper device using a plurality of vacuum cups in which a sensor associated with each vacuum cup senses contact between the associated vacuum cup and an object. An output signal from the sensor then causes vacuum to be applied to the vacuum cup. Only those vacuum cups where contact with a workpiece or object is detected by the associated sensor have vacuum applied thereto. However, this control apparatus still does not detect a decrease in vacuum level within the vacuum cup after vacuum has been initially applied to the vacuum cup or reapply vacuum to the vacuum cup after the initial application of vacuum to maintain the vacuum at a preset level.
Thus, it would be desirable to provide a control apparatus for vacuum cups which overcomes the problems associated with previously devised vacuum control apparatus. It would also be desirable to provide a control apparatus for vacuum cups which detects the vacuum level in the vacuum cup so as to cease the application of vacuum when the vacuum level reaches a preset level and which reapplies vacuum to the vacuum cup when a decrease in vacuum below a predetermined amount is detected.